Endocardial leads, used for cardiac pacing, cardioversion and monitoring, are placed through a transvenous route to position one or more sensing and/or stimulation electrodes in a desired location within a heart chamber or interconnecting vasculature. Routing an endocardial lead along a desired path to a target implant site can be difficult and is dependant upon the physical characteristics of the lead. One type of lead includes a plurality of individually insulated wires or filars formed in a coil that extends from the proximal to the distal end of the lead. Each electrode carried by the lead is electrically connected to one of the wires.
Advancements in medical technology have led to the use of devices for imaging and for therapy that involve exposing a patient to energy fields that may be disruptive to implantable medical devices. For example, magnetic resonance imaging generates cross-sectional images of a human body using nuclear magnetic resonance (NMR). The MRI process begins with positioning a body to be imaged in a strong, uniform magnetic field, which polarizes the nuclear magnetic moments of protons within hydrogen molecules in the body by forcing their spins into one of two possible orientations. Then an appropriately polarized radio-frequency field, applied at resonant frequency, forces spin transitions between these orientations. The spin transitions create a signal, an NMR phenomenon, which can be detected by a receiving coil.
Traditionally, the use of magnetic resonance imaging has been discouraged for patients with implantable medical devices because the energy fields generated during operation of the MRI may interfere with or affect the performance of the IMD. Particularly, RF fields generated by magnetic resonance imaging might cause conductors used in the leads of an IMD to heat up, causing local burns in the body.
The tendency of lead electrodes to heat up when placed in a RF field is related to the inductance of the lead coil, because wound conductors are particularly susceptible to excitation from RF fields. Generally, it is desirable for a lead coil to have increased inductance in order to minimize effects from RF fields generated during magnetic resonance imaging.
Therefore, it is desirable to have an IMD lead that is capable of carrying multiple electrodes, has good handling and endo-vascular mobility, is small in diameter (i.e. less than 7 French (˜0.9185 inches)) and can be used in an MRI environment.